In many electronic applications, electrical resonators are used. For example, in many wireless communications devices, radio frequency (rf) and microwave frequency resonators are used as filters to improve reception and transmission of signals. Filters typically include inductors and capacitors, and more recently resonators.
As will be appreciated, it is desirable to reduce the size of components of electronic devices. Many known filter technologies present a barrier to overall system miniaturization. With the need to reduce component size, a class of resonators based on the piezoelectric effect has emerged. In piezoelectric-based resonators, acoustic resonant modes are generated in the piezoelectric material. These acoustic waves are converted into electrical waves for use in electrical applications.
One type of piezoelectric resonator is a bulk acoustic wave (BAW) resonator. Typically, there are two types of BAW resonators: a Film Bulk Acoustic Resonator (FBAR) and a solidly mounted bulk acoustic resonator (SMR). Both the FBAR and the SMR comprise acoustic stacks that are disposed over a reflective element. The reflective element of an FBAR is a cavity, normally in a substrate over which the acoustic stack is mounted. The reflective element of an SMR is a Bragg reflector comprising alternating layers of high acoustic impedance and low acoustic impedance layers.
The BAW resonator has the advantage of small size and lends itself to Integrated Circuit (IC) manufacturing tools and techniques. The FBAR includes an acoustic stack comprising, inter alia, a layer of piezoelectric material disposed between two electrodes. Acoustic waves achieve resonance across the acoustic stack, with the resonant frequency of the waves being determined by the materials in the acoustic stack.
Generally, a bulk acoustic wave (BAW) resonator has a layer of piezoelectric material between two conductive plates (electrodes), which may be formed on a thin membrane. The piezoelectric material may be a thin film of various materials, such as aluminum nitride (AlN), zinc oxide (ZnO), or lead zirconate titanate (PZT), for example. Thin films made of AlN are advantageous since they generally maintain piezoelectric properties at high temperature (e.g., above 400° C.). However, AlN has a lower piezoelectric coefficient d33 than both ZnO and PZT, for example.
In FBAR (film bulk acoustic resonators) devices, strain sensors, mechanical oscillators, and other electronic and microelectromechanical systems (MEMS) devices, it can be necessary to keep the device isolated mechanically and chemically from its surrounding environment. For example, the performance of an FBAR device is severely degraded if the motional device is in contact with the overmold compound of a typical microelectronics package. To this end, many devices have complicated and expensive encapsulation processes and methodologies.
What is needed, therefore, is a structure that overcomes at least the shortcomings of known structures described above.